The 3rd-Generation Partnership Project (3GPP) is developing standards for several Coordinated Multipoint (CoMP) techniques, including a CoMP mode for downlink transmissions known as “coherent joint transmission”. According to this technique, multiple transmission points simultaneously transmit signals carrying the same information streams to a single mobile terminal (“user equipment” or “UE” in 3GPP terminology), where each transmission point is using the same time-frequency resources. The signals from the different points are intended to be combined constructively at the air interface level (i.e., prior to any demodulation or detection). The UE is unaware of this joint transmission, and does not need to take special action when demodulating the data blocks that are cooperatively transmitted by multiple points.
In order to maximize the combining gain from coherent multipoint transmission, the signals from the different points should reach the UE at almost exactly the same time. For example, in the context of a Long-Term Evolution system, which uses Orthogonal Frequency Division Multiplexing (OFDM) for downlink transmissions, differences in arrival time at the UE should be smaller than the cyclic prefix length of an OFDM symbol.
Furthermore, the carrier frequencies used by the different transmission points should be exactly the same or within a rigorous difference range, so that such a difference will not impair the constructive combining gain at UE. Likewise, the phase of the signals from the different points should be aligned in a manner such that the signals from different points are additively combined.
FIG. 1 illustrates a simple example of CoMP transmission, where two transmission points 100a and 100b transmit nearly identical signals to UE 110. Because the transmission delays between the UE and each of the transmission points is not necessarily identical, coordination is needed to ensure that the time-of-arrival (relative to a given sequence of symbols) is the same, or very nearly the same, for the two signals. If so, and if the carrier frequencies are tightly synchronized, the signals will add constructively in the radio front-end of UE 110, yielding a better quality received signal than would have been received if only one of the two transmission points was transmitting the signal.
There are a variety of possible network deployments for CoMP in general and coherent joint transmission in particular. These include scenarios that involve a macro-cell scenario, such as the remote deployment of macro base stations, and heterogeneous network scenarios, sometimes referred to as “HetNet.” A HetNet can be constructed using various combinations of different types of nodes, for instance, the macro/micro/pico nodes, relay/repeater nodes, etc. FIG. 2 illustrates a coherent joint transmission for a simple example of a HetNet deployment. In FIG. 2, macro node A, micro node B, and pico node C transmit nearly identical signals to UE 110. Again, the transmissions are coordinated so that the signals arrive at UE 110 at very nearly the same time.
Another scenario in which signal timing and frequency synchronization can be important is in the context of a system that uses a distributed antenna system (DAS). In this system, a radio base station “owns” (i.e., controls) one or more remote radio units (RRUs). Each RRU has its own oscillator to generate the radio frequency carrier signal. In the DAS system, frequency synchronization between
RRUs is very important in order to achieve gains from coherent joint transmission, just as it is in the HetNet or other multiple transmission point scenario. Accordingly, cost-effective techniques for ensuring frequency synchronization between coordinating transmission points are needed.